1. Field of the Invention
The invention relates to a relative position detecting apparatus which is mounted in a movable object and detects the relative position of that movable object with respect to another moving object, as well as to a relative position detecting system made up of three or more movable objects each of which is provided with the relative position detecting apparatus.
2. Description of the Related Art
As related art, apparatuses that observe for satellite signals, obtain the observed data, and calculate the position of a movable object by performing a positioning calculation are used in vehicle navigation systems, marine vessels, and mobile phones and the like.
The main method of calculating the position is referred to as single point positioning. Single point positioning is a method which calculates pseudo ranges to a plurality of satellites based on the observed data obtained at a single observation point, and then calculates the position of the observation point according to the principle of triangulation.
However, a known method referred to as interferometric positioning that enables the position to be more precisely calculated is also being put into practical use. Interferometric positioning is a method which calculates the position of a measure point by obtaining a base vector from a known point to the measure point based on the observed data obtained at the observation point. Interferometric positioning uses a carrier phase with a short cycle compared to a C/A code that is normally used in single point positioning. Therefore, the frequency (i.e., the integer bias) from the satellite to the observation point must be specified.
Japanese Patent Application Publication No. 2003-270319 (JP-A-2003-270319), for example, describes an invention relating to this interferometric positioning. More specifically, JP-A-2003-270319 describes a method for calculating satellite positioning that involves the following steps. That is, for a satellite signal from the same satellite from which signals are received at a position on a reference side and a position on a positioning side (i.e., a measuring side), a relational expression between i) a carrier phase difference, ii) a carrier phase bias of an integer included in the carrier phase difference, iii) a phase difference between the positioning side and the reference side related to a reference phase by observation of the carrier phase, iv) the position of the satellite sending the satellite signal received by the reference side and the positioning side, and v) the positions of the reference side and the positioning side, is obtained. When relational expressions obtained at a plurality of observation timings are simultaneously satisfied, a conditional expression that defines the difference in the reference phase and the position on the positioning side is obtained and the condition obtained by this conditional expression is then substituted into the relational expression, such that an expression in which the variables corresponding to the difference in the reference phase and the position on the positioning side have been eliminated is obtained. Then the integer bias is obtained from the conditions that are satisfied with only a slight difference for this expression with no variables.
This method described in JP-A-2003-270319 is a method for calculating the integer bias between two points. However, when it is applied to a case in which three or more movable objects measure their relative positions, various problems arise. For example, when each of three or more movable objects measures its relative position with respect to the other movable objects, the communication and calculation loads increase. As a result, it may no longer possible to coordinate the positional relationships among the plurality of movable objects.